Modern aircraft, both commercial and military, are migrating to more electric architectures that integrate the main and auxiliary engine start functions with the electric power generating equipment. Conventional brushless, wound field synchronous generators are among the candidate machines for this new class of starter/generator and are the logical choice for modern variable frequency (VF) alternating current (AC) electric system architectures. These modern aircraft starter/generators represent a class of variable speed motor drives in the start mode of operation and use solid-state power converters to process typically high potential direct current (DC) electric power into VF AC electric power suitable for driving the variable speed AC electric machine. Typical of all variable speed synchronous motor drives, the position of the motor rotor is required to control the solid-state power converter to meet motor performance requirements.
A resolver mounted to the starter/generator rotor provides this required rotor position information but it has been identified as imposing size, weight, and complexity or unreliability penalties. Thus, it is desirable to provide the electric start function in a self-sensing or sensorless manner, that is, without a resolver or other overt rotor position sensing means. Additionally, these starter/generators are sized for worst case starting conditions that may require a plurality of machines to start a single main engine at cold day conditions. It is thus required in some applications to parallel a multiplicity of starter/generators to provide full rated torque from each of these starter/generators at standstill.
Many sensorless schemes have been invented over the past 20 to 30 years to enable sensorless operation of many different classes of electric motors under a variety of different operating conditions. At rotor standstill or low speed there is insufficient back electromotive force (EMF) generated in a dynamoelectric machine to enable an accurate estimate of rotor position using only passive measurement of terminal potentials and currents. Some means must be provided to stimulate the machine in order to extract rotor position information. Many alternative schemes have been reported in the technical literature over the years.
Stimulation can be applied to either the rotor or the stator, it can be either transient or continuous, and it may be of different frequencies. Nonetheless, all the reported approaches require some means to stimulate the machine and some means to interpret or demodulate the stimulation response in order to provide an estimate of the rotor position. One advantageous approach is described in U.S. Pat. No. 5,585,709 by Jansen et al., herein incorporated by reference.
Jansen et al. describes a carrier injection sensorless (CIS) method of estimating the position and velocity of the rotor of a dynamoelectric machine. CIS works by applying a high frequency excitation signal with an electrical current or potential rotating waveform to the dynamoelectric machine at a high enough frequency that it sweeps around the stator faster than the rotor is turning, thus “viewing” the rotor from all angles. This “viewing” is manifested in measuring the resulting rotating current or potential waveform, which contains information about the rotor due to rotor position dependent differences in the equivalent magnetic circuit of the dynamoelectric machine.
If the rotating current waveform at the machine terminals is transformed to its stationary two-axis equivalent (αβ) and x-y plotted, a non-circular orbit is seen that rotates with the rotor. This is the electromagnetic image of the dynamoelectric machine and in general, each machine design has its own unique image. This technique works with any dynamoelectric machine that has rotor saliencies that result in a change in impedance as seen at the stator windings to the high frequency excitation signal.
Although the CIS technique described in Jansen et al. allows estimation of the position of a rotor pole, either “north” or “south”, it does not inherently allow the determination of which pole's position is estimated. This could give rise to 180 electrical degree errors in position estimation, and such error can be very undesirable.
A means for north-south pole determination using the CIS technique for wound field synchronous machines is described in a pending patent application by Markunas et al., U.S. Ser. No. 10/930,629, filed 31 Aug. 2004, owned by the assignee of this application and incorporated by reference. In Markunas et al., the means for north-south pole discrimination exists in the field current rectification provided by the rotating rectifier in the field circuit of the wound field synchronous machine. In essence, the rotating rectifier diodes produce an asymmetry in the effective d-axis impedance as viewed from the stator. The d-axis looks different when the rotating rectifier is forward biased than when it is reverse biased. This rectification induces second and higher order harmonics in the d-axis currents for carrier injection. This asymmetry only exists when the wound field synchronous machine is unexcited. As soon as excitation is applied, the rotating rectifier is forward biased, eliminating the asymmetry.
Markunas et al. utilises the positively rotating components (+2ωct) of the carrier second harmonics that contain additional rotor position information before field excitation. This information is used as a north-south pole indicator to determine if the CIS system has locked onto a north or south pole of the rotor. One problem with this technique is that the best carrier frequency for determining a north pole from a south pole is in the range of approximately 50 to 150 Hz for high power aircraft brushless synchronous starter/generators such as for aircraft of new design. The best carrier frequency for the remaining start sequence is significantly higher, on the order of 300 to 1200 Hz.